Conventionally, radar apparatuses having a TT (Target Tracking) function to track a target object based on echo signals received by a radar antenna are known. The TT function is a function to detect a transition in position of a target object around a ship based on a transition in radar image obtained before a current timing. With this TT function, for example, it is possible to display a target object having a possibility of colliding against the ship, by assigning a given mark thereto.
JP2004-309246A discloses a radar apparatus having the TT function. This radar apparatus sets a given range rearward of a ship and, once a target object being tracked enters this range, stops the tracking of the target object.
JP2000-304853A stores target object data obtained before a current timing and, based on this target object data, calculates a current velocity vector of a specific target object specified by a user.
JP2014-235040A discloses a radar apparatus which automatically starts tracking a target object determined to have a collision risk against a ship. This radar apparatus determines the collision risk based on a distance to the target object, a speed of the target object, a CPA (Closest Point of Approach), etc. This speed of the target object is obtained based on a change in position of the target object obtained in every scan.
In JP2014-235040A, whether to start the tracking is determined based on the speed of the target object. However since the direction of the speed of the target object is not taken into consideration in JP2014-235040A, the target object is treated the same regardless of it moving toward or away from the ship. Moreover, in JP2000-304853A and JP2014-235040A, since the speed of the target object is calculated by using the positional change of the target object, it is required to determine radar echoes in previous and latest scans which are based on the same target object, thus the processing amount increases.